In certain radio frequency (RF) communications devices, a diplexer is used to allow for full-duplex waveform communication. A diplexer is a passive circuit device that allows both a lower frequency signal and a higher frequency signal to coexist on a common signal port (e.g., a shared antenna port).
One example configuration in which a diplexer is used in a mobile RF device is set forth in U.S. Pat. No. 8,340,712 to Rofougaran. This patent discloses a method and system for utilizing a diplexer/duplexer for WCDMA operation as a filter for supporting GSM-based operation. A receiver portion of a wireless device may receive WCDMA signals and GSM signals via at least one duplexer or diplexer. The duplexers may enable bandpass filtering of received GSM signals. The wireless device may enable receiving the WCDMA signals and the GSM signals via a single antenna coupled to the duplexers. The receiver portion may also enable selecting a processing path for received WCDMA signals or a processing path for received GSM signals. The receiver portion may enable amplification and filtering of the WCDMA signals. Filtering of the WCDMA signals may be performed via surface acoustic wave (SAW) filters. The receiver portion may also enable filtering of the GSM signals via the duplexers and amplification of the filtered GSM signals.
Generally speaking, a relatively high Q filter may be needed in a diplexer to achieve the requisite bandwidth, selectivity, and insertion loss. In this regard, ceramic resonators are sometimes used due to their high Q, stability and repeatability. At higher frequencies (>1 GHz), the size of the ceramic resonators is relatively small. However, at lower frequencies (e.g., in the ultra high frequency (UHF) range), the size of the ceramic resonators may be relatively large compared to the overall size of a handheld radio. In either case, the requisite footprint of such diplexers (or, more generally, RF filters) may make them difficult to incorporate in a mobile form factor, and thus further enhancements in RF filter/diplexer design may be desirable in certain applications.